参考stackoverflow，可行的方法为
一、使用glCopyImageSubData，这个方法最直观而且简单。不过需要 OpenGL 4.3
二、为目标texture创建FBO。使用shader，采用texture2D把源texture画到目标texture
三、为源texture创建FBO。使用glCopyTexSubImage2D从framebuffer拷贝到目标纹理

    fboId = srcTex->GetFrameBufferId();
    glBindFramebuffer(GL_FRAMEBUFFER, fboId);
    outTexId = dstTex->GetTextureId();
    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, outTexId);
    glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0,
        0, 0, srcTex->GetWidth(), srcTex->GetHeight());
    glBindTexture(GL_TEXTURE_2D, 0);
    glBindFramebuffer(GL_FRAMEBUFFER, 0);
四、glBlitFramebuffer。这个需要OpenGL ES 3.0以上支持
glGenFramebuffers(1, &fbo);  // 源texture和目标texture共用一个FBO即可
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, 
                       GL_TEXTURE_2D, tex1, 0);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, 
                       GL_TEXTURE_2D, tex2, 0);
glDrawBuffer(GL_COLOR_ATTACHMENT1);
glBlitFramebuffer(0, 0, width, height, 0, 0, width, height, 
                  GL_COLOR_BUFFER_BIT, GL_NEAREST);